1. Field of the Invention
The present invention relates to a suspension controller, especially to a suspension controller which alters the suspension characteristic suitably for the condition of a road on which vehicle moves.
2. Description of the Related Art
Recently, a suspension controller has been proposed in which the road condition is detected by a vehicle height sensor which detects the change in the distance between a vehicle axle and the body. During running on a rough road, the suspension characteristic or the vehicle height is altered for damping the vibration of the vehicle body for attaining good controllability and stability as well as good riding comfort. An example of such an invention as described above is found in Published Unexamined Japanese Patent Application No. 57-172808 entitled "Vehicle Height Adjusting Apparatus". In this application, when the road condition is changed from smooth to rough, the suspension controller is operated to increase the vehicle height by more than a predetermined value so as to prevent bottoming of the vehicle attributable to the bounding of the vehicle. The above prior art detects the rough road if the determination is made that the actual vehicle height exceeds a predetermined reference value and such case occurs more frequent than usual. Accordingly, vehicle height is altered to higher position that is suitable for running on a rough road.
However, the suspension controller according to the prior art possesses the following problems.
(1) Vehicle vibration arising when the vehicle moves over continuous rough road, e.g., the vibration characterized as pitching or bouncing, usually includes vibration derived by superimposing the resonant vibration of sprung mass (frequency of 1.2-1.3 Hz) on the frequency of the resonant vibration of unsprung mass (about 15 Hz). The prior art is operated to detect the suspension characteristic altering condition on the basis of the counted number of times that the actual vehicle height surpassed the average vehicle height by more than a predetermined value within a predetermined time interval. However, the above-mentioned predetermined time interval is not set considering the cycle time of the resonant vibration of sprung mass. Therefore, the determination of the condition of vibration being exerted on the vehicle body cannot be made, unless the vehicle moves on a road surface for more than the predetermined time interval.
(2) Due to the problem mentioned in paragraph (1), the alteration operation of the suspension characteristic is delayed because it takes a long time to determine the vibration condition. Moreover, once the vibration as mentioned above occurs, it is impossible to eliminate it until the suspension characteristic is altered, thus giving an uncomfortable feeling to the passengers. In addition to bad riding comfort, such vibration alters the vehicle attitude. Accordingly, the drivability and stability of the vehicle are deteriorated.